The present invention relates to a method for manufacturing a capacitor element used in a solid electrolytic capacitor comprising a powder of a valve-acting metal such as tantalum, niobium, or aluminum, a method for manufacturing a solid electrolytic capacitor using such a capacitor element, and a solid electrolytic capacitor using such a capacitor element.
As described, for example, in Japanese Patent Applications Laid-open Nos. H7-74062 and H10-50563 representing the conventional technology, a capacitor element for a solid electrolytic capacitor of this type is manufactured by employing a method in which:
(1) first, a porous anode chip body obtained by solid molding a powder of a valve-acting metal and then sintering is manufactured so that an anode wire manufactured from a valve-active metal bonded to the metal powder protrudes from the anode chip body;
(2) then, the anode chip body is immersed in a conversion solution such as an aqueous solution of phosphoric acid and in this state an anode oxidation treatment is conducted by applying a direct current between the metal powder and the conversion solution, thereby forming a dielectric film such as a tantalum pentoxide film on the surface of each metal particle in the anode chip body;
(3) then several cycles of operations are repeated, each cycle comprising the steps of immersing the anode chip body into an aqueous solution for solid electrolyte such as an aqueous solution of manganese nitrate, impregnating the porous structure in the anode chip body with the aqueous solution for solid electrolyte such as an aqueous solution of manganese nitrate, pulling the anode chip body from the aqueous solution for solid electrolyte, drying, and firing. As a result, a solid electrolyte film composed of a metal oxide such as manganese dioxide is formed on top of the dielectric film on the surface of the anode chip body;
(4) then, a cathode-side electrode film composed of a graphite layer as a prime layer and a metal layer, e.g., of silver as a top layer is formed on the surface of the solid electrolyte layer in the anode chip body.
Further, the capacitor element manufactured in the above-described manner has been conventionally assembled into a packaged solid electrolyte capacitor by disposing the capacitor element between an anode-side lead terminal and cathode-side lead terminal so that the anode wire thereof is mounted in a fixed condition on the anode-side lead terminal and the cathode-side electrode film is connected to the cathode-side lead terminal, and then sealing the entire configuration with a package body.
As described hereinabove, the basic configuration has been conventionally produced by mounting the anode wire in a fixed condition on the porous anode chip body so that the anode wire was electrically joined to the metal powder in the anode chip body. As a result, the formation of a dielectric film, a solid electrolyte layer, and a cathode-side electrode film on the anode chip body could be easily conducted in a state in which the anode chip body was supported with the anode wire. Another advantage was that in the case where a solid electrolytic capacitor was obtained as a final product, the anode-side lead terminal could be reliably electrically connected to the metal powder in the anode chip body via the anode wire.
However, the following drawbacks were associated with this method.
Thus, the capacitor element was obtained by forming a dielectric film, a solid electrolyte layer, and a cathode-side electrode film on the anode chip body in a state in which an anode wire was mounted in a fixed condition on the anode chip body, and this capacitor element was assembled with the anode wire mounted in a fixed condition thereon in a solid electrolytic capacitor as a final product. Therefore, the size of the anode wire was added to the size of the solid electrolytic capacitor. As a result, when the size of the solid electrolytic capacitor as a final product has been determined in advance, the anode wire hindered the increase in the capacitance of the capacitor, and when the capacitance of the capacitor has been determined in advance, the anode wire increased dimensions and weight of the solid electrolytic capacitor.
Moreover, in order to connect electrically the anode wire in the capacitor element to the metal powder in the anode chip body, the one end portion thereof was embedded in the anode chip body. Therefore, the effective volume taken by the metal powder in the volume of the anode chip body was reduced by the aforementioned end portion of the anode wire embedded in the anode chip body. This was a significant obstacle for reducing the size and increasing the capacity of the capacitor element.